Heavy fighting has erupted between Syrian forces and rebel fighters in the Golan Heights, as the United Nations seeks the return of 44 peacekeepers seized there last week.

The clashes occurred near the Quneitra border crossing, the same area where al-Qaida-linked al-Nusra Front rebels abducted the peacekeepers. It is not clear whether the Syrian army has taken control of the crossing from the militants.

The Britain-based Syrian Observatory for Human Rights said there were casualties from Monday's fighting, but did not say how many.

The peacekeepers are part of the U.N. Disengagement Observer Force that monitors the buffer zone in the Golan Heights between Syria and Israel. The militants abducted 44 peacekeepers from Fiji and surrounded 75 peacekeepers from the Philippines on Thursday. The Filipino troops escaped after their position came under fire.

On Sunday, the United Nations said it did not have information on where the abducted peacekeepers were being held, but was "actively seeking their immediate and unconditional release."